1. Field of the Invention
The present invention relates to fuel injection pumps used in diesel engines and other internal combustion engines, and more particularly to a fuel injection pump that is a variable-pre-stroke type made so that a preparatory injection (pre-injection) is performed near exhaust top dead center in a diesel engine.
2. Description of the Related Art
Conventionally, in diesel engines of the direct injection type in which fuel is directly injected into the combustion chambers, a small quantity of fuel is injected preparatorily (pre-injected) in the vicinity of opposite phase of the main injection, that is, before the cam angle becomes approximately 180 degrees (near top dead center in the exhaust stroke), for the purpose of reducing nitrogen oxides (NOx).
More specifically, the purpose of this pre-injection is to create a condition wherein the fuel is in an atomized state near top dead center in the compression stroke of the engine, thereby sharply reducing main injection ignition lag and reducing NOx. Examples of this are to be seen in laid-open Util. Mod. Ap. No. S63-121772 1988!, laid-open Util. Mod. Ap. No. 63-191266 1988!, and laid-open Pat. Ap. No. H6-117341 1994!, etc.
However, if the fuel injection pump is of the accumulator type which uses something like a common rail, the necessary pressure can be secured with any desired timing, which makes it comparatively easy to implement this kind of two-stage injection employing main injection and pre-injection. In a jerk-type fuel injection pump in which a plunger is caused to reciprocate up and down in a plunger barrel, it is very difficult to implement two-stage injection while making the pre-injection volume variable, and a structure for implementing this is desired.
It is also desirable that two-stage injection be implemented with which the timing of the pre-injection relative to the main injection can be freely controlled.